High pressure gas atomization (hereafter HPGA) is described in Anderson et al. U.S. Pat. Nos. 5,125,574 issued Jun. 30, 1992, and 5,228,620 issued Jul. 20, 1993. HPGA has shown considerable promise as in making very fine metal and alloy powder having a rapidly solidified particle microstructure. These patents describe the atomization parameters required to effectively use the kinetic energy of the supersonic gas jet streams discharged from an atomizing nozzle to disintegrate a melt into ultrafine, generally spherical powder particles. In particular, HPGA in accordance with those patents employs an atomization nozzle having multiple discrete, circumferentially spaced straight-bored gas jet discharge orifices or passages arranged about a nozzle melt supply tube having a melt discharge orifice. The melt supply tube has a central melt discharge orifice and adjacent 45 degree frusto-conical surface defining a melt supply tube tip. High pressure inert gas is supplied to the gas nozzle manifold at a high enough pressure (e.g. 1050 psig) and discharged from the jet passages at a 45 degree gas jet apex angle to establish a subambient pressure region at the melt discharge orifice to create an aspiration effect or condition that draws melt out thereof for atomization at an apex edge region on the adjacent frusto-conical tip surface. The discharged gas jet streams atomize the melt and form a narrow, supersonic spray containing very fine melt droplets that solidify rapidly as powder particles that are collected.
In addition to gas pressure, certain gas jet orifice or passage geometries/dimensions of the patented HPGA nozzle have been found to be important in achieving satisfactory atomization of the melt in the HPGA regime of operation. For example, the straight-bored gas jet discharge orifices or passages typically provide an impinging gas jet apex angle of 45 degrees with improved tangency of the gas jets relative to the 45 degree frusto-conical surface adjacent to the melt discharge orifice. The improved tangency of the gas jets provides enhanced laminar flow of the gas jets across the frusto-conical surface of the melt supply member.
Supersonic convergent-divergent nozzle design is a known technology for large rocket motors. However, boundary layer formation is inevitable to confined gas flow phenomena. The formation of an attached quiescent boundary layer is due to gas friction against the nozzle wall. Aerospace engineers and mechanical engineers have believed that it is impossible to develop a viable miniature supersonic nozzle because the boundary layer thickness would become significant relative to the total available gas flow crosssectional area. For example, a first order aerodynamic approximation derived from incompressible fluid flow theory shows that the throat area, for a nominal throat diameter of 0.029 inch, will be reduced by 63% from the formation of a boundary layer against the adjacent nozzle wall. Such overall boundary layer created by compressible flow would be significant in reducing the effective throat area.
It is an object of the present invention to provide improved high pressure gas atomizing nozzles and methods using convergent-divergent discrete gas microjet discharge orifice or passage configurations that are viable to provide supersonic gas jets for atomizing molten material.
It is another object of the present invention to provide improved high pressure gas atomizing nozzles and methods using multiple discrete gas microjet discharge orifice or passage configurations that impart increased gas kinetic energy to the molten material being atomized to improve atomization thereof.